DESCRIPTION: The goal is to continue to develop and employ cultured mammalian cell systems with which to quantify all of the kinds of mutations induced by ionizing radiations, and to illuminate mechanisms of their induction. The rationale is that since mutations underlie genetic diseases including many cancers, in vitro assays can help predicting risks and shed light on processes of carcinogenesis in vivo. The focus will be 137Cs-gamma radiation, as a model agent at various regimens of exposure, e.g. high and low dose rates. Mutation analysis will be carried out in different constructs of AL human-hamster hybrid cells in which a spectrum of mutations from 100,000 to 1.4 x 108 base pairs of DNA can be quantified and resolved by Southern and PCR analysis. The focus will be on chromosomal mutations which appear to be the principal mutation induced by ionizing radiations. Each specific aim is designed to characterize the role in mutagenesis (both in terms of the numbers and kinds of mutants induced) of different cellular properties that affect mutagenesis: gene linkage and ploidy, cell cycle phase, DNA repair (of both double strand breaks and base damage), and p53 status (mutant vs wild type). They propose also to clone the MIC1 gene that encodes the S1 antigen which is the principal marker gene employed in the assay so as to be able to study intragenic mutations.